A safety input system for monitoring a sensor can include a voltage sensing circuit configured to produce a first detection output upon sensing a signal exceeding a voltage threshold, and a current sensing circuit configured to produce a second detection output upon sensing the signal exceeding a current threshold and/or voltage threshold. The signal can come from a sensor in an industrial automation system, such as a light curtain. The first detection output can be provided to a first processor, and the second detection output can be provided to a second processor, so that two processors can independently monitor the sensor. The signal can be monitored by separate input systems. Periodically the first processor can inject a first test signal into the input system for the second processor to detect, and the second processor can inject a second test signal into the input system for the first processor to detect.